Most Abundant Soils

The properites of soils summarized in soil units mapped by the Natural
Resources Conservation Service can be used to asess limitations for
septic systems. The Soil Units map is not shown because the map
cannot be presented in a readable fashion. Those interested in
soil unit names should look at the soil survey for Gallatin County
available at the Gallalatin County office of the Natural Resource
Conservation Service. There are ten themes that relate to
limintations:

Example Questions the Database Can Answer

1. Where are the areas where septic limitations are most severe?
The
severity
index is a score. The score was built by assigning a 3 to
severe, a 2 to
moderate
and
a 1 to slight and a 0 to everything else. The
scores for all
restrictive
features
are added to produce the total score in SEVINDX.
There are eight
are
eight
restrictive features so the maximum possible score is 3 X 8 =
24.
The
highest
score in the
LWQD
is
17.
2. How many severe ratings are present in each soil map
unit?
In
this
theme the number of severe ratings for the eight restrictive
features is tallied
for
each
soil map unit and reported. The largest number of severe
limitations is four.
3. What are the septic limitations from the perspective
of flooding?
4. What are the septic limitations from the perspective
of depth to bedrock?
5. What are the septic limitations from the perspective
of ponding water?
6. What are the septic limitations from the perspective
of depth to high water table?
7. What are the septic limitations from the perspective
of percolation rate for drain fields?
8. What are the septic limitations from the perspective
of filtration of microbes?
9. What are the septic limitations from the perspective
of slope?
10. What are the septic limitations from the perspective of large
stones (coarse fragments)?
11. Where are the areas with hydric soils?
12. Where are the high and low runoff areas?
This data is useful for runoff analysis with the NRCS Curve Number
approach.
13. Where are the areas with high potential for steel corrosion
which might impact underground pipes?
14. Where are the areas with high potential for concrete
corrosion
which might impact concrete septic tanks?
15. Where are the areas with high potential for frost action?
16. Where are the areas with high SAR where drainfield soils
may be sensitive to sodium from water softeners?
17. Where are the areas with shrink-swell clays which may be
sensitive
to sodium from water softeners?
18. What soil units exist in the area I am interested in?

This theme focuses on most abundant soils mapped by the NRCS and
US Forest Service. Each soil map unit may contain more than one
soil.
The most limiting soil was selected by examining each soil in a map
unit
for severe ratings
in the order listed in
U.S. Natural Resources Conservation Service, 1996, National Soil
Survey Handbook, Part 620 NRCS 620-46 Soil Interpretations
Rating
Guides, Table 620-17 Septic Tank Absorption Fields.
The first soil that produced a severe listing was used for all further
limitations. For example, if there were three soils and none had
severe flooding limitations, but one had depth to bedrock less than 40
inches, that soil became the soil used to define the most limiting
restrictions.
Likewise, if none of the three soils had a severe flooding
limitation
or depth to bedrock limitation, but one had a ponding limitation, the
soil
with the ponding limitation was selected to represent the most limiting
conditions regarding installation and operation of septic
systems.
The choice of soil was independent of the area the soil component
represents
in the map unit. As a result the most limiting soil may represent
as little as ten percent of the mapped unit. The
examples
above illustrate questions that can be explored with this
database.
The limitations are sometimes surmountable with sound engineering
practice,
but such work can be expensive and can substantially influence the
development
potential of the site from a cost perspective. In all cases, the
user must be aware that site specific work may reveal other limitations
or show that the map unit has areas without the indicated
limitations.
These themes simply alert the user to problems which may be present
based on second and third order soil surveys.

About the Data

Mstabnd.shp Metadata
The limitations are based upon Natural Resources and Conservation
Service
(NRCS) data for the most abundant soil mapped at a scale of 1:24,000
during
a level second order soil survey and Gallatin National Forest data for
the most abundant soil mapped at a scale of 1:62.500 during a third
order
survey. The data are projected in UTM meters NAD83. The
limitations
were developed from NRCS 620-46 Part 620
Soil
Interpretations Rating Guides (Table 620-17). The
most abundant soil may represent from 33 to 100 percent of the land
area
in the mapping unit since many of the soils represent a complex. The
limitations
for the most abundant soil differ from that for the most limiting
soil.
This data should not be used for site assessment but is useful for
planning
and generalized assessment of areas larger than10-20 acres.

The theme for lowland agricultural soils was created from soil
tables in Rolfes, T., J. Brooker, B. Duncan, T. Keck, and R. Simms,
1998,
SSURGO Soil Survey Geographic Database, Gallatin County Area,
Montana:
USDA-NRCS, Bozeman, Montana. The theme for the upland forest
soils
was developed from Davis, C.E., and Shovic, H.F., 1984 Soils survey of
Gallatin national Forest Area, Southwestern Montana: Interim
Draft
Report: USDA-Forest Service, Gallatin National Forest; and from Davis,
C.E., and Shovic, H.F., 1996, Soil Survey of Gallatin national Forest,
Montana: USDA-Forest Service, Gallatin National Forest, Bozeman,
Montana. The soils in the Big Sky area
have been extrapolated from US Forest Service soil data; more detailed
surveys have been conducted in this area that are not yet available
digitally, but are available through the Gallatin NRCS office.
The Gallatin National Forest soils were not originally interpreted for
septic limitations, but such interpretations can be made as shown by
Christner,
W.T., Jr., 1999, Septic Interpretations from a Third Order Soil
Survey:
Master of Science in Land Resources and Environmental Science, Montana
State University, Bozeman, Montana. Each soil unit may contain as
many as three soils. The most abundant soil was selected,
septic
system limitation criteria
were applied to
the soil properties, and severe, moderate, slight, or no data
were
assigned to each property for each most abundant soil. The most
abundant
soil in each unit covers from 33-100 percent of the soil unit's area.